True; not only the challenger usually has a higher chance of winning than the odds suggest, but if the wager spreads over a month or more, that's a lot of $$ stuck into one place.
Often in the meantime wagering options arise somewhere and you can't take advantage.
I usually don't like parlaying big favs, altough i cashed a nice one a month ago ( dimitrenko, taylor and haye, all taking place on saturday)